Generally, contact stress is generated as gears are interlocked, and thereby generating fitting and the like on the surface of a gear part. Accordingly, durability of the gear parts may be reduced and noise may occur. Particularly, when load applied to a gear is increased with such a gear which is downsized or minimized and a powertrain has high output, the above-mentioned problems get serious. Due to these durability problems, there have been attempts to improve durability, for example, providing steels with improved fitting resistance, performing a carbonitriding treatment or adding a shot-peening process. However, heavily loaded gears also may cause problems during endurance test.
For example, when the carbonitriding treatment is employed, fitting resistance characteristic can be improved but bending strength is reduced. Moreover, when the shot-peening process is added to the manufacturing process, bending strength can be improved, but fitting resistance characteristic is reduced. Although a high carburizing method is an innovative method in the art by improving the fitting resistance characteristic and the bending strength at the same time, the conventionally performed high carburizing method may not be applied to manufacturing of gears because of deep hardening-depth, brittleness due to formation of carbide in the form of network/coarse, and severe thermal degradation during heat treatment.
Recently, a transmission gear has been produced using steel material having improved fitting resistance or by modified carbonitriding heating method. This steel grade has elevated high-temperature softening resistance due to high contents of silicon (Si) and molybdenum (Mo), and quenching property thereof is improved by increased chromium (Cr) content. In some cases, the modified carbonitriding method has been used for increasing the fitting resistance by increasing the amount of residual austenite and has been used for manufacturing a gear. The conventional high carburizing method has been employed for increasing wear resistance by forming carbide on the surface. However, it is difficult to apply to a gear due to large carbide size and deep hardening-depth. Further, gear tooth may be deformed significantly to cause noise and durability problems when such high carburizing method is applied.
The description provided above as a related art of the present invention is just for helping understanding the background of the present invention and should not be construed as being included in the related art known by those skilled in the art.